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Instrumentation and Measurement

Precision Signal Processing

Circuit Function & Benefits

The circuit shown in Figure 1 measures peak and rms power
at any RF frequency from 450 MHz to 6 GHz over a range of
approximately 45 dB. The measurement results are converted
to differential signals in order to eliminate noise and are
provided as digital codes at the output of a 12-bit SAR ADC
with serial interface and integrated reference. A simple twopoint
calibration is performed in the digital domain.

The ADL5502 is a mean-responding (true rms) power detector
in combination with an envelope detector to accurately
determine the crest factor (CF) of a modulated signal. It can be
used in high frequency receiver and transmitter signal chains
from 450 MHz to 6 GHz with envelope bandwidths over 10 MHz.
The peak-hold function allows the capture of short peaks in the
envelope with lower sampling rate ADCs. Total current
consumption is only 3 mA @ 3 V.

The ADA4891-4 is a high speed, quad, CMOS amplifier that
offers high performance at a low cost. Current consumption is
only 4.4 mA/amplifier at 3 V. The amplifier features true singlesupply
capability, with an input voltage range that extends 300 mV
below the negative rail. The rail-to-rail output stage enables
the output to swing to within 50 mV of each rail, ensuring
maximum dynamic range. Low distortion and fast settling time
makes it ideal for this application.

The AD7266 is a dual, 12-bit, high speed, low power, successive approximation ADC that operates from a single 2.7 V to 5.25 V power supply and features sampling rates up to 2 MSPS. The device contains two ADCs, each preceded by a 3-channel multiplexer, and a low noise, wide bandwidth track-and-hold
amplifier that can handle input frequencies in excess of 30 MHz.
Current consumption is only 3 mA at 3 V. It also contains an
internal 2.5 V reference.

The circuit operates on a single +3.3 V supply from the
ADP121, a low quiescent current, low dropout, linear regulator
that operates from 2.3 V to 5.5 V and provides up to 150 mA
of output current. The low 135 mV dropout voltage at 150 mA
load improves efficiency and allows operation over a wide input
voltage range. The low 30 μA of quiescent current at full load
makes the ADP121 ideal for battery-operated portable equipment.

The ADP121 is available in output voltages ranging from 1.2 V
to 3.3 V. The parts are optimized for stable operation with small
1 μF ceramic output capacitors. The ADP121 delivers good
transient performance with minimal board area.
Short-circuit protection and thermal overload protection
circuits prevent damage in adverse conditions. The ADP121 is
available in tiny 5-lead TSOT and 4-ball, 0.4 mm pitch halidefree
WLCSP packages and utilizes the smallest footprint
solution to meet a variety of portable applications.

Circuit Description

The RF signal being measured is applied to the ADL5502.
A single 75 Ω termination resistor at the RF input in parallel
with the input impedance of the ADL5502 provides a
broadband match of 50 Ω. More precise resistive or reactive
matches can be applied for narrow frequency band use (see
the RF Input Interfacing section of the ADL5502 data sheet).

The internal filter capacitor of the ADL5502 provides averaging
in the square domain but leaves some residual ac on the output.
Signals with high peak-to-average ratios, such as W-CDMA or
CDMA2000, can produce ac residual levels on the ADL5502
VRMS dc output. To reduce the effects of these low frequency
components in the waveforms, some additional filtering is
required. The internal square-domain filter capacitance of the
ADL5502 can be augmented by connecting a CFLTR capacitor
between Pin 1 (FLTR) and Pin 2 (VPOS). The ac residual can be
reduced further by adding capacitance to the VRMS output.
The combination of the internal 100 Ω output resistance and
the added output capacitance produces a low-pass filter to
reduce output ripple of the VRMS output (see the Selecting the
Square-Domain Filter and Output Low-Pass Filter section of the
ADL5502 data sheet for more details).

To measure the peak of a waveform, the control line (CNTL)
must be temporally set to a logic high (reset mode for >1 μs)
and then set back to a logic low (peak-hold mode). This allows
the ADL5502 to be initialized to a known state. When setting
the device to measure peak, peak-hold mode should be toggled
for a period in which the input rms power and crest factor (CF)
is not likely to change.

If the ADL5502 is in peak-hold mode and the CF changes from
high to low or the input power changes from high to low, a
faulty peak measurement is reported. The ADL5502 simply
keeps reporting the highest peak that occurred when the peakhold
mode was activated and the input power or the CF was
high. Unless CNTL is reset, the PEAK output does not reflect
the new peak in the signal.

The ADL5502 is capable of sourcing a VRMS output current of
approximately 3 mA. The output current is sourced through the
on-chip, 100 Ω series resistor; therefore, any load resistor forms
a voltage divider with this on-chip resistance. It is
recommended that the ADL5502 VRMS output drive high
resistive loads to preserve output swing. If an application
requires driving a low resistance load (as well as in cases where
increasing the nominal conversion gain is desired), a buffering
circuit is necessary.

The PEAK output is designed to drive 2 pF loads. It is
recommended that the ADL5502 PEAK output drive low
capacitive loads to achieve a full output response time. The
effects of larger capacitive loads are particularly visible when
tracking envelopes during the falling transitions. When the
envelope is in a fall transition, the load capacitor discharges
through the on-chip load resistance of 1.9 kΩ. If the larger
capacitive load is unavoidable, the additional capacitance can be
counteracted by putting a shunt resistor to ground on the PEAK
output to allow for fast discharge. Such a shunt resistor also
makes the ADL5502 run higher current, and it should not be
lower than 500 Ω.

Typical measured performance characteristics of the circuit are
presented in Figure 2 through Figure 5.

The turn-on time and pulse response is strongly influenced by
the size of the square-domain filter (CFLTR) and output shunt
capacitor connected to the VRMS output. Figure 6 (taken from
the ADL5502 data sheet) shows a plot of the output response to
an RF pulse on the RFIN pin, with a 0.1 μF output filter
capacitor and no square-domain filter capacitor (CFLTR). The
falling edge is particularly dependent on the output shunt
capacitance.

To improve the falling edge of the enable and pulse responses,
a resistor can be placed in parallel with the output shunt
capacitor. The added resistance helps to discharge the output
filter capacitor. Although this method reduces the power-off
time, the added load resistor also attenuates the output (see the
Output Drive Capability and Buffering section of the ADL5502
data sheet). Figure 7 (taken from the ADL5502 data sheet)
shows the improvement obtained by adding a parallel 1 kΩ
resistor.

The RMS and PEAK outputs of the ADL5502 pass through
unity gain buffers that drive cross-coupled stages for converting
the single-ended outputs to differential signals. The internal
+2.5 V reference of the AD7266 (via the DCAPA and DCAPB pins) passes through another unity gain buffer and a voltage divider. This sets the common-mode voltage of the network to +1.25 V.

The AD7266 achieves simultaneous samples of the RMS and
PEAK outputs and transfers the data within a 1 μs response
time. The data is provided on a single serial data line. Because
slope and intercept vary from device to device, board-level
calibration must be performed to achieve high accuracy. In
general, calibration is performed by applying two input power
levels to the ADL5502 and measuring the corresponding output
voltages. The calibration points are generally chosen to be
within the linear operating range of the device. The best-fit line
is characterized by calculating the conversion gain (or slope)
and intercept using the following equations:

where:

VIN is the rms input voltage to RFIN.
VVRMS is the voltage output at VRMS.

Once gain and intercept are calculated, an equation can be
written that allows calculation of an (unknown) input power
based on the measured output voltage.

For an ideal (known) input power, the law conformance error of
the measured data can be calculated as

Figure 8 and Figure 9 show plots of the VRMS and PEAK error
at 25°C, the temperature at which the ADL5502 is calibrated.
Note that the error is not zero; this is because the ADL5502
does not perfectly follow the ideal linear equation, even within
its operating region. The error at the calibration points is,
however, equal to zero by definition.

When the characteristics (slope and intercept) of the VRMS and
PEAK outputs are known, the calibration for the CF calculation
is complete. A three-stage process must be taken to measure
and calculate the crest factor of any waveform. First, the
unknown signal must be applied to the RF input, and the
corresponding VRMS level is measured. This level is indicated
in Figure 10 as VVRMS-UNKNOWN. The RF input, VIN, is calculated using VVRMS-UNKNOWN and Equation 3.

Next, the CW reference level of PEAK, VPEAK-CW, is calculated
using VIN (that is, the output voltage that would be seen if the incoming waveform was a CW signal).

Finally, the actual level of PEAK, VPEAK-UNKNOWN, is measured and the CF can be calculated as

where VPEAK-CW is used as a reference point to compare
VPEAK-UNKNOWN. If both VPEAK values are equal, then the CF is 0 dB, as shown in Figure 11 with the CW signal (taken from the ADL5502 data sheet). Across the dynamic range, the calculated CF hovers about the 0 dB line. Likewise, for complex waveforms
of 3 dB, 6 dB, and 9 dB CFs, the calculations accurately hover
about the corresponding CF levels.

Common Variations

For applications that require less RF detection range, the
AD8363 rms detector can be used. The AD8363 has a detection
range of 50 dB and operates at frequencies up to 6 GHz. For
non-rms detection applications, the AD8317/AD8318/AD8319
or ADL5513 can be used. These devices offer varying detection
ranges and have varying input frequency ranges up to 10 GHz
(see CN-0150 for more details).

Circuit Evaluation & Test

This circuit uses the EVAL-CN0187-SDPZ circuit board and the
EVAL-SDP-CB1Z System Demonstration Platform (SDP)
evaluation board. The two boards have 120-pin mating
connectors, allowing for the quick setup and evaluation of the
circuit’s performance. The EVAL-CN0187-SDPZ board contains
the circuit to be evaluated, as described in this note, and the
SDP evaluation board is used with the CN0187 evaluation
software to capture the data from the EVAL-CN0187-SDPZ
circuit board.

​Equipment Needed

PC with a USB port and Windows® XP or Windows Vista®
(32-bit), or Windows® 7 (32-bit)

EVAL-CN0187-SDPZ circuit evaluation board

EVAL-SDP-CB1Z SDP evaluation board

CN0187 evaluation software

Power supply: +6 V, or +6 V “wall wart”

RF signal source

Coaxial RF cable with SMA connectors

Getting Started

Load the evaluation software by placing the CN0187 Evaluation
Software disc in the CD drive of the PC. Using "My Computer,"
locate the drive that contains the evaluation software disc and
open the Readme file. Follow the instructions contained in the
Readme file for installing and using the evaluation software.

Functional Block Diagram

See Figure 1 of this circuit note for the circuit block diagram,
and the “EVAL-CN0187-SDPZ-SCH” pdf file for the circuit
schematics. This file is contained in the CN0187 DesignSupport Package.

Setup

Connect the 120-pin connector on the EVAL-CN0187-SDPZ
circuit board to the connector marked “CON A” on the
EVAL-SDP-CB1Z evaluation (SDP) board. Nylon hardware
should be used to firmly secure the two boards, using the holes
provided at the ends of the 120-pin connectors. Using an
appropriate RF cable, connect the RF signal source to the
EVAL-CN0187-SDPZ board via the SMA RF input connector.
With power to the supply off, connect a +6 V power supply to
the pins marked “+6 V” and “GND” on the board. If available, a
+6 V "wall wart" can be connected to the barrel jack connector
on the board and used in place of the +6 V power supply.
Connect the USB cable supplied with the SDP board to the USB
port on the PC. Note: Do not connect the USB cable to the mini
USB connector on the SDP board at this time.

Test

Apply power to the +6 V supply (or “wall wart”) connected to
EVAL-CN0187-SDPZ circuit board. Launch the evaluation
software and connect the USB cable from the PC to the USB
mini-connector on the SDP board. The software will be able to
communicate to the SDP board if the Analog Devices System
Development Platform driver is listed in the Device Manager.

Once USB communications are established, the SDP board can
now be used to send, receive, and capture serial data from the
EVAL-CN0187-SDPZ board.

The data in this circuit note were generated using a Rohde &
Schwarz SMT-03 RF signal source and an Agilent E3631A
power supply. The signal source was set to the frequencies
indicated in the graphs, and the input power was stepped and
data recorded in 1 dB increments.

Information and details regarding how to use the evaluation
software for data capture can be found in the CN0187
Evaluation Software Readme file.

Information regarding the SDP board can be found in the
SDP User Guide.

Pricing displayed is based on 1-piece. The USA list pricing shown is for budgetary use only, shown in United States dollars (FOB USA per unit), and is subject to change. International prices may vary due to local duties, taxes, fees and exchange rates.

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